Respiratory insufficiency is an abnormality of the respiratory system with an adverse effect on gas exchange, resulting in a lowering of the arterial partial pressure of oxygen, an increase in the arterial partial pressure of carbon dioxide, or both. It may be due to a decrease in ventilation in the presence of normal lungs, a lung abnormality, or a combination of these.
Respiratory insufficiency generally results in worse gas exchange during sleep, for a variety of reasons including decreased respiratory drive and postural factors. The decrease in voluntary muscle function during REM sleep may cause a marked worsening of respiratory function during this stage of sleep, depending on the importance of the accessory muscles of respiration in the particular patient.
Sleep may be associated with varying degrees of upper airway obstruction, referred to as obstructive sleep apnoea (OSA).
Sleep-disordered breathing (SDB) generally refers to types of breathing disruption that occur during sleep. The most common form of sleep-disordered breathing is obstructive sleep apnea (OSA). Loud, intermittent snoring, apneas, and hypopneas characterize OSA.
Respiratory insufficiency, and possibly OSA, occur in conjunction with conditions experienced by patients with chest wall, neuromuscular, amyotrophic lateral sclerosis (ALS), or lung disease, such as chronic obstructive pulmonary disease (COPD). Because the symptoms of sleep apnea present themselves as a result of a precursor, SDB has become the general term used to describe any disease state that manifests apneas and/or hypopneas during sleep. Apneas and hypopneas interfere with gas exchange, fragment sleep, and frequently cause oxygen desaturations. In severe cases, patents may experience these oxygen desaturations and arousals from sleep hundreds of times each night.
The most common treatment of OSA is to administer continuous positive airway pressure (CPAP). CPAP was invented by Sullivan and taught in U.S. Pat. No. 4,944,310. Briefly stated, CPAP treatment acts as a pneumatic splint of the airway by the provision of a positive pressure, usually in the range 4-20 cm H2O. The air is supplied to the airway by a motor driven blower whose outlet is coupled by an air delivery hose to a nose (or nose and/or mouth) mask sealed with the patient's face. An exhaust port is provided in the delivery tube proximate to the mask.
Ventilatory assistance may be provided by bi-level ventilators, proportional assist ventilators and servo-controlled ventilators. Each type of ventilator utilizes different methods for assisting with patient respiration and achieves different goals.
Such ventilator devices provide appropriate responses to the changing conditions of the patient. For example, ventilatory devices determine when to trigger and cycle varying pressure levels associated with inspiratory and expiratory support so that the device will synchronize with the respiratory cycle of the patient. Triggering is the event associated with the initiation of the pressure levels intended for the patient's inspiration. Cycling is the event associated with switching to the pressure levels intended for the patient's expiration. Also, the devices may provide some method for increasing or decreasing ventilation during periods of hypoventilation or hyperventilation respectively. Maximizing machine performance in either or both of these areas generally results in greater patient comfort and better treatment of respiratory insufficiency.
Simple bi-level ventilators provide a higher pressure during the inspiratory portion of the patient's breathing cycle, a so-called IPAP, and a lower pressure during the expiratory portion of the breathing cycle, a so-called EPAP. Traditionally, the switching may be accomplished by monitoring the respiratory flow or pressure and defining a threshold level. When the measured value exceeds the threshold, the device will trigger the IPAP pressure. When the measured value falls below the threshold, the device will cycle to the EPAP pressure. Other alternatives to such switching involve recorded respiration rates and the monitoring of elapsed time from the start of either inspiration or expiration. The machine may switch to the following portion of the respiratory cycle, either inspiration or expiration, after reaching the expected time for the previous part of the respiratory cycle.
A different servo-ventilator device developed by ResMed Ltd. accomplishes synchronization by delivering smooth cyclical pressure changes based on a calculated instantaneous phase. Embodiments of the apparatus are the subjects of commonly owned U.S. patent application Ser. No. 09/661,998 and U.S. Pat. Nos. 6,532,957 and 6,532,959, the disclosures of which are incorporated herein by reference. Generally, the apparatus provides an instantaneous mask pressure P(t) based upon a fraction of the patient's airway resistance R (this fraction ranging from zero to a substantial value but less than 1), respiratory airflow f(t), an amplitude A, and an estimation of the patient's instantaneous respiratory phase ϕ as applied to a pressure waveform template π(ϕ) as follows:P(t)=PG+Rf(t)+Ax(ϕ) for all f(t)  (inspiration and expirationwhere:A=G∫(V(t)−VTGT)dt AMIN<A<AMAX, andPo is an initial pressure.
In this type of ventilator, V(t) can be, for example, one half the absolute value of the respiratory airflow f(t). The ventilation target VTGT may be a percentage of a measured volume of airflow, e.g., 95% of the average minute volume or a preset prescribed minute volume. G is the gain of the integral servo-controller, values in the range 0.1-0.3 cmH2O per L/min error in ventilation per second being suitable. AMIN and AMAX are limits set on the degree of support A for comfort and safety. The limits 0.0 and 20.0 cmH2O respectively are generally suitable.
In detecting the patent's respiratory phase, the apparatus uses a respiratory airflow signal and its derivative as input data for a set of fuzzy logic rules that are associated with particular phases of respiration. Using the results of the evaluations of the rules, a single continuous phase variable is derived and used as the instantaneous respiratory phase. This phase value as applied to a pressure waveform template then proportionally varies the delivered pressure in a manner that generates a realistic and comfortable respiratory cycle. Simultaneously, the calculation of A based on the target ventilation VTGT guarantees a desired level of ventilation.
With regard to the issue of synchronization, while determining continuous instantaneous phase has its benefits in detecting the patient's respiratory cycle, it may be desirable to use a method for determining the pressure to be delivered to a patient which does not use a simple function π(ϕ) to determine the number by which A in the above equation is multiplied. It may also be beneficial to utilize alternative methods for triggering the initiation of inspiratory pressure.
With regard to the control of a pressure response to changing patient respiratory needs, in the above equation the rate of change of pressure support is simply proportional to the difference between the target ventilation and the actual ventilation. It may be desirable for small errors to result in a gentler response, and in particular for marked hypoventilation to result in a more brisk response, since it is much more likely to result in significant hypoxia. By contrast, when measured ventilation is well above target, it may not be necessary or desirable to briskly decrease the ventilatory support level, particularly considering that when there has been a sudden change in leak in a noninvasive ventilatory system, the measured ventilation is almost always greater than the actual ventilation until the leak estimation system has substantially compensated for the change in leak.
A problem with ventilators that target total ventilation (traditionally called “minute ventilation”) is that total ventilation may not provide an accurate measure of the extent to which the actual needs of the patient are met. Such a measure does not account for the fact that patients have anatomical and physiological deadspace, and that the deadspace varies between patients. In particular, with a particular total ventilation, a high respiratory rate and a low tidal volume will provide lower alveolar ventilation than a low respiratory rate and a high tidal volume. A device which servocontrols minute ventilation is thus at risk of providing inadequate alveolar ventilation at high respiratory rates.